The present invention relates to supercharger cooling and in particular to cooling a hotter end of a supercharger including two or more rotating rotors.
Modern roots supercharger have improved efficiency by having an axial inlet at an inlet end and timing gears at an opposite end. Unfortunately, the opposite end is hotter than the inlet end exposing the timing gears to such heat reduced gear and lubricant life.
Twin screw type superchargers draw air into the rear of the supercharger and compress the air as it travels from the rear to the front of the supercharger between supercharger rotors. According to the ideal gas law, the air traveling through the supercharger is heated proportional to the compression of the air inside the supercharger and is thus hotter at the front of the supercharger then at the rear of the supercharger. Further, no supercharger is 100 percent efficient, and although screw type superchargers are more efficient than roots-type superchargers, they remain approximately 70 to 80 percent efficient, which means that if the ideal temperature increase is 100 degrees, the actual temperature increase in 20 to 30 percent greater (in terms of absolute temperature). This temperature variation from the front and the rear of the supercharger results in a corresponding unequal heating of supercharger components, and as a result, unequal expansion of the supercharger components and an accompanying variation in clearances (for example, rotors, cases, front plate, gears, bearings, and the like) between supercharger components. The rotor bearing are interference fit, and when the end cover becomes hot enough, the bearing may rotate in the bearing seats, damaging the seats, and causing the rotors to contact and destroy the supercharger.
When the front plate expands from heat, gears positioned by the front plate experiences an increased gear clearance. Correct gear positions are critical in a twin screw supercharger because the gear positions determine the location of the male and female rotors and their separation. Excessive gear clearance may also result in rotor contact, and proper operation of the supercharger requires that the rotors remain in phase with each other throughout the operating temperature range of the supercharger, which is between 100° F. and 450° F.
A possible solution to the variation of clearances with temperature is to increase rotor to rotor clearance to compensate for the temperature variation over the entire temperature range of supercharger operation. Unfortunately increasing the clearances in a twin screw type supercharger reduces supercharger efficiency. Further, increasing gear clearance results in noisy supercharger operation which is often objectionable to a driver, and accelerates wear of the gears.
Further, the rotors of twins screw type superchargers are generally made from aluminum. The aluminum rotors generally have 0.003 inches to 0.004 inches of clearance and thus controlling the expansion of the rotors, regardless of the clearances between gears, has been an issue with the twin screw type superchargers for decades. Greater than ideal clearances have been incorporated into the supercharger designed to deal with rotor expansion. Unfortunately these large clearances reduce supercharger efficiency resulting in hotter air charges, lower output, and higher power requirement for operating the supercharger. Further, should the rotors contact each other due to excessive expansion, the supercharger is generally destroyed.
The front (output) or discharge side of the supercharger is the hottest and rotor contact always occurs towards the front of the supercharger. The rear (inlet) or intake is ingesting cooler ambient air so there is generally no rotor contact at the rear end of the supercharger. And, the higher the temperatures inside the supercharger, the more severe the rotor contact and the farther the contact reaches from the rear to the front of the supercharger.
The rotors fore and aft shafts and bearings support and stabilize the positions of the rotors. Unfortunately, the front plate having a higher temperature expands more than the rear plate which is closer to ambient air temperature. This temperature imbalance accompanied by the expansion imbalance causes the front of the rotors to separate more than the rear of the rotors. The rotor gears are attached to the front of the rotors and as a result experienced increased gear lash as the fronts of the rotors separate. Both the gear lash and the rotor expansion move the rotors outward closer to the supercharger case and the timing change from the excess gear lash results in circumferentially excess movement of one rotor or in relation to the other.
In addition to loss of efficiency and damage to the supercharger, the increased temperatures shorten the life of supercharger seals.
The front case of the supercharger contains the oil used to lubricate the gears and bearings. Friction from the rotating gears, bearings, and seals heat the oil, and higher supercharger rpm, greater boost, and higher air temperature at the front of the supercharger, further contribute to higher oil temperature. These effects combine to make controlling the temperature of the twin screw supercharger extremely difficult.
A possible solution to cooling the supercharger is to provide a pressurized flow of engine oil to the supercharger gears. Unfortunately, this approach requires external lines to provide a source of pressurized oil to the supercharger, and external drain lines from the supercharger to the engine oil pan to drain the oil from the supercharger, which create potential oil leaks. Further, additional heating of engine oil raises oil temperature and affects oil flow reducing the cooling affect of the oil.
Thus, a need remains for cooling the front (output) end of a screw type supercharger.